A typical prior art hole cutter, also known as hole saw, has a circular base with a cylindrical blade body extending axially therefrom, and defining a plurality of cutting teeth or saw teeth on the cutting edge of the blade body. The base typically has a central hole, often threaded, and multiple pin drive holes around the central hole for engaging an arbor for driving the hole cutter. Example of drive arbors are provided in U.S. Pat. No. 7,073,992, which is incorporated by reference in its entirety.
Hole cutters are often fabricated by forming the base in the form of a circular cap and then attaching, such as by welding, the blade body. Such caps are often formed either by machining plate material or by draw-forming sheet material. With a plate-type cap, the arbor hole is formed through the thickness of the plate and threaded for engagement with the arbor. The cap defines a peripheral flange and a recess for receiving the blade body. The blade body is attached, such as by welding, to the cap. In order for the cap to have sufficient strength, and the arbor hole to have sufficient length so that the threaded length of the hole is sufficient for secure engagement to the arbor, the plate material is at least about 0.187″ thick. Examples of plate cap hole cutters are shown in U.S. Pat. Nos. 6,939,092 and 7,264,428, which are incorporated by reference in their entireties. While plate cap hole cutters possess good strength and dimensional accuracy, the thick plate material entails material cost and the machining process for forming the recess and arbor and drive holes requires expensive machining equipment. The machining process is also relatively time consuming.
A sheet-type cap is formed using a sheet material, such as sheet metal, that is no more than about 0.111″ thick. The peripheral rim of the cap is formed by drawing the edges of the sheet metal blank into cup-like shape, and machining a recess into the peripheral edge of the cup for attachment to the blade body. The drawing process involves using pulling/tensile forces applied to the sheet metal to flow and stretch the material into the desired shape. The thinness of the sheet material does not permit the arbor hole to be threaded. Therefore, the arbor hole is also drawn inwardly (in the same direction as the cup-shaped portion) to form a hole of sufficient length to be threaded for arbor engagement.
Drawn sheet metal caps are less costly in both material and manufacturing costs than plate-type caps, in part because sheet material is thinner than plate. However, sheet metal cap hole cutters have several performance disadvantages compared to plate-type caps. As they are made from thinner material, they are not as rigid as plate-cap hole cutters and subject to stress and deformation during cutting due to cutting forces. These forces are most severe at the arbor hole and deformation of the cap can cause undesirable arbor wobble. This is a particular issue with larger diameter hole cutters. In addition, the edges of the drawn arbor hole at the external side of the cap are rounded as a result of the drawing process, and the arbor does not set flush against the cap and/or the internal diameter of the arbor hole, potentially contributing to flex or arbor wobble. Moreover, the draw-forming process is less precise, making attaching the blade body potentially more difficult and resulting in a dimensionally less true hole cutter (out of round).